RE: Trim Question - Needing left rudder on downline at idle  
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All Forums >> RC Airplanes >> Pattern Flying >> RE: Trim Question - Needing left rudder on downline at idle
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RE: Trim Question - Needing left rudder on downline a... - 8/30/2008 2:59:58 AM   
TonyF


 

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From: Rosamond, CA, USA
Status: offline 		All sounds good in theory, but in practice you can trim a model to not need rudder mix. Believe me, all of my models don't have it and all would track in insides, outsides, vertical up and down. Just get the design and trim correct.

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RE: Trim Question - Needing left rudder on downline a... - 8/30/2008 7:02:17 AM   
OhD


 

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quote:

ORIGINAL: ual767

Consider this:
A motor's thrust line (right thrust added) exists to counteract torque, p-factor, spiral slipstream etc. It would only make sense that the right thrust built into a plane will only be right at a certain thrust level. All of the rest of the time, the thrust angle is wrong. Torque, etc are constantly changing but the right thrust angle stays the same. Cessna 152 needs right rudder in a high power climb and LEFT rudder in an idle descent if you want to keep the nose straight and not induce a slip. The motor is set up for no rudder needed in level cruise flight. Same with pattern planes , you can't have a plane with a fixed thrust angle be correct at all thrust settings.



Hey Chris, I believe what you are describing are the effects of p-factor. Here is an explanation for the vertical dive. I believe that my propeller is adding DRAG not thrust on the downline. Given that, and the fact the motor axis is negative with respect to the vertical, I believe the the p-factor puts more drag on the right side of the airplane. Hold a propeller up and look at it from one tip with it at a negative angle to the vertical (like downthrust). You will see the right blade is flatter and therefore will create more drag. Looks like a mix is a smart fix. Now the fact that the motor axis is also not aligned with the vertical in yaw due to the right thrust it would seem we also need some downtrim due to p-factor. How's zat?

Jim O

(in reply to ual767)
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RE: Trim Question - Needing left rudder on downline a... - 8/31/2008 5:59:20 AM   
patternflyer1



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Status: offline 		Can you stop your prop with a brake on your controller? If so, it may make an interesting test to stop it on a downline.

C

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RE: Trim Question - Needing left rudder on downline a... - 8/31/2008 5:39:29 PM   
OhD


 

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Status: offline 		With my present brake setting the prop will not stop but it definitely lowers the rpm and slows down the rate of descent. I'm not sure what a stopped prop would do as you'd never know where it stopped. It still might be an interesting experiment.

Now I want everyone to tell the truth. Did you add a mix, throttle to rudder? I did.

Jim O

(in reply to patternflyer1)
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RE: Trim Question - Needing left rudder on downline a... - 8/31/2008 6:30:48 PM   
patternflyer1



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Status: offline 		I have no rudder to throttle mix. I did for a while, then I added more right thrust in the Abbra and my uplines and downlines look good and feel good. Well, in my opinion. lol

As for stopping the prop, well IMO if you are using a brake to slow down by keeping the prop spinning at a lower rpm, I could possibly see drag you speak of. But if the prop was stopped and not spinning at all, would that go away? Just a thought. I don't use a brake. Possibly why I don't have this issue with the down lines.. Maybe?

C

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RE: Trim Question - Needing left rudder on downline a... - 9/2/2008 5:58:20 AM   
woodie


 

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From: Placerville, CA, USA
Status: offline 		Thanks to all for your input, observations, questions, etc. Sorry for the delay in responding, I was out of town and Frank's laptop was not working correctly.

Taking all the input into account, I am going to try some different thrust settings to see if that will help alleviate some of the trim offsets without sacrificing any other 'good' stuff. Luckily, all I have to do to go back to 'normal' is remove any shims I may have put in place. Will let you all know if I find anything interesting.

Woodie (Don)

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< Message edited by woodie -- 9/2/2008 6:00:20 AM >

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RE: Trim Question - Needing left rudder on downline at... - 9/4/2008 12:53:20 AM   
OhD


 

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quote:

ORIGINAL: woodie

Ok, Woodie here (aka Don Atwood).

I have a trimming or aerodynamic question for you aero gurus out there. All my recent pattern planes (all electric) have needed left rudder trim in downlines to stay straight. With the rudder at neutral, there is a noticeable right yaw at idle. Once you apply a little power, the yaw diminishes until it is pretty much gone at half power. My plane have been a Brio (very noticeable), a friends Brio (same issue), Integral (2) and a Genesis.

I have read the glo power Brios had the same issue so I don't know if it is primarily an electric power issue but it sure is noticeable on my models.

Any ideas or good explanations? Am I trimming wrong to create this? (or maybe I am just crazy) ;-)

thanks
Woodie


OK Woodie, (aka Don)

I flew with the low throttle to rudder mix today. It might be my imagination but I believe it fixed all the maneuvers I was having trouble with. Well maybe not all. Anyway, thanks for starting this thread. Have you got anymore insights you'd like to share?

Ohd, (aka Jim O)


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RE: Trim Question - Needing left rudder on downline a... - 9/4/2008 12:36:25 PM   
rwlewis


 

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quote:

ORIGINAL: iflyrctoo

OK Mark,

Time to disign a variable thrust angle into the next version of your Pentathlon.

Joe Dunnaway


Just saw this comment...The solution to the thrust angle issue is to simply build a Voodoo Express. It flies perfectly straight with zero right/left and Zero Down/up....

Richard

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RE: Trim Question - Needing left rudder on downline at... - 9/4/2008 2:22:21 PM   
woodie


 

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quote:

ORIGINAL: OhD


quote:

ORIGINAL: woodie

Ok, Woodie here (aka Don Atwood).

I have a trimming or aerodynamic question for you aero gurus out there. All my recent pattern planes (all electric) have needed left rudder trim in downlines to stay straight. With the rudder at neutral, there is a noticeable right yaw at idle. Once you apply a little power, the yaw diminishes until it is pretty much gone at half power. My plane have been a Brio (very noticeable), a friends Brio (same issue), Integral (2) and a Genesis.

OK Woodie, (aka Don)

I flew with the low throttle to rudder mix today. It might be my imagination but I believe it fixed all the maneuvers I was having trouble with. Well maybe not all. Anyway, thanks for starting this thread. Have you got anymore insights you'd like to share?

Ohd, (aka Jim O)




I flew the Genesis with more right thrust in it yesterday and the mix set on a switch. While it changed the mix requirements, it didn't have any positive affect on the downline, still needed left rudder. I will try a little more right thrust today....... Stay tuned, news at 10PM... ;-)

Woodie (aka Don)


< Message edited by woodie -- 9/4/2008 2:24:22 PM >

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RE: Trim Question - Needing left rudder on downline ... - 9/4/2008 4:15:37 PM   
David Bathe



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From: Oslo, NORWAY
Status: offline 		But it's very normal to need left rudder at idle. Even when I removed the throttle to rudder mix and changed over to right thrust built into the engine, I still kept left rudder at idle.
It kicks out after two clicks above.

It's straight forward... you need the right thrust... or right trim... or right rudder mix to combat the effect of the propellers natural slip stream that hits the left hand side of the fin/rudder thus causing the swing left. ...Thus the right thrust, right trim etc!

Minimalize that effect, ie. throttle way back and the in built trim will cause the swing right!
Mix in a little left rudder to come in two or three clicks above idle. Problem fixed.

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RE: Trim Question - Needing left rudder on downline ... - 9/5/2008 3:15:03 AM   
woodie


 

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From: Placerville, CA, USA
Status: offline 		OK, here's the news from the testing today. I added more right thrust ( a little over 1/2 degree) and it didn't seem to affect the downlines at all. Still needed left rudder mix on downlines. I thought maybe I would need to offset the uplines with slight left rudder trim but it didn't seem to need any change for flight above about 1/3 throttle. I will try reducing right thrust next time.

Don

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RE: Trim Question - Needing left rudder on downline ... - 9/5/2008 11:01:43 AM   
ofremmi



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From: Horten, NORWAY
Status: offline 		Sir's
It might be that I'm sloppy with my set-ups, but why dont you just trim the rudder for a perfect downline, and then adjust the thrust-line to achieve straigh lines going up ? The issue that thrust offset generates a yaw during braking might be theoretically correct, but my guess is that the effect are so small that it will be hard to detect.



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RE: Trim Question - Needing left rudder on downline ... - 9/5/2008 8:20:46 PM   
OhD


 

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quote:

ORIGINAL: ofremmi

Sir's
It might be that I'm sloppy with my set-ups, but why dont you just trim the rudder for a perfect downline, and then adjust the thrust-line to achieve straigh lines going up ? The issue that thrust offset generates a yaw during braking might be theoretically correct, but my guess is that the effect are so small that it will be hard to detect.




I was thinking the opposite. Set the motor/engine at zero and use mixes for everything; up, down, knife edge, etc.

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RE: Trim Question - Needing left rudder on downline ... - 9/5/2008 9:58:39 PM   
ual767



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From: Chantilly, VA, USA
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quote:

ORIGINAL: ofremmi

Sir's
It might be that I'm sloppy with my set-ups, but why dont you just trim the rudder for a perfect downline, and then adjust the thrust-line to achieve straigh lines going up ? The issue that thrust offset generates a yaw during braking might be theoretically correct, but my guess is that the effect are so small that it will be hard to detect.




The problem is that any fixed thrust angle will be wrong most of the time - this is particularly so with prop changes on electric motors. You don't want to have to change the motor's thrust angle every time you change props. It is much easier as Jim (OhD) says to mix away the yaw. Ideally, you can set your plane up in the tx as different models depending on the prop and set the mixes accordingly then just change models when you change props and you will have the right mixes already done. Granted, this will take many flights to get right but when you are done, the throttle changes will be completely invisible in the yaw axis.
If you mechanically trim for downlines, you will have way too much left rudder trim flying around other 95% of the flight and require even more right rudder mix to cancel out the left offset in the rudder.

< Message edited by ual767 -- 9/5/2008 10:01:12 PM >

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